Add JSDoc comments to all Beszel PocketBase API types explaining the
cryptic short field names (cpu, mu, du, dr, ns, b, etc.), their units
(bytes, bytes/s, percentages), and the distinction between ns/nr
(all-interface traffic) vs b (public interface bandwidth).
Also documents the mapToSystemRow mapping and chart data extractors
so future readers understand the data flow without needing Beszel
source code.